This section provides background information related to the present disclosure which is not necessarily prior art.
For many applications in spectroscopy, e.g. for process monitoring, microscopy, or microspectroscopy, rapidly (<1 ms) and distantly (>2000 cm−1) tunable coherent light sources, hereinafter referred to as laser systems, are desired. These laser systems should either have spectrally particularly narrow-band pulses for high resolution (<0.3 THz or 10 cm−1), and thus temporally long pulses (>1.5 ps), or they should have particularly short pulses (<500 fs) with a broad spectral bandwidth (>0.8 THz) for a high peak output. In addition, the construction of these laser systems should be as robust as possible. Following the construction of a laser system, there should be no degrees of freedom for adjustment, such as using adjustable mirrors, so that operation by the user is possible without any adjustment required. In addition, this should prevent maladjustment due to environmental influences. These laser systems should be able to be used routinely in a wide variety of industrial and clinical settings without it being necessary to have special expertise with lasers.
Normally a combination of a pump laser and an optical parametric oscillator (OPO) or amplifier (OPA) is used to generate ultrashort light pulses having an adjustable wavelength. According to the current state of the art, as a rule the pump laser for the OPO or OPA is based on a mode-coupled laser oscillator that generates ultrashort laser pulses at a fixed or tunable wavelength. In an OPO, parametric generation converts a pump pulse to two wavelength-shifted pulses, specifically a signal pulse and an idler pulse. One of the two pulses (idler pulse or signal pulse) is fed back via a resonator, so that it then acts as a seed pulse for parametric amplification. Since parametric amplification is an energy-conserving process, at the same time the non-resonant signal pulse/idler pulse is further amplified, so that this pulse may be decoupled as output signal of the laser system. The decoupled pulse shall be referred to hereinafter as the output pulse.
If wavelength-tunable, ultrashort pulses having a duration of less than 500 fs (broad spectrum) are to be generated, these laser systems are always constructed using the free beam technique and are frequently based, e.g., on titan-sapphire lasers. If ultrashort pulses having a duration in the picosecond range (narrow-band spectrum) are to be generated, the laser systems used are either also based on free beam technique or, more recently, nearly completely on optical fibers, e.g. fiber optical parametric oscillators (FOPO). However, these new, largely tunable FOPOs also require a free beam portion, even if it is just a small free beam portion, that requires adjustment and represents a weakness for maintenance-free operation.
Coherent laser systems having free beam regions are generally more complex than completely glass fiber-based systems. In addition, the adjusting, stabilization of the adjustment, and mechanical stability requirements for the device housing mean much greater complexity. However, robust, maintenance-free laser systems comprising glass fiber components completely welded thereto are desired for broad distribution and use of such laser systems.
Another disadvantage in the prior art is that changing from the emission of picosecond pulses to femtosecond pulses is not possible in any of the laser systems available in the past or is only possible with significant modifications to the laser system. A simple change from femtosecond operation to picosecond operation, e.g. by exchanging individual modules, which would also entail significant advantages for production and use of such laser systems, has not been possible in the past. In addition, there is the drawback in the FOPOs known from the prior art that the selection of the wavelength of the emitted laser radiation in most of the concepts is limited solely to mechanical or thermal tuning mechanisms due to limitations in the pump laser. However, due to mass inertia or temperature inertia, these mechanisms are slow in principle (>1 ms per wavelength step) and therefore moreover permit only continuous tuning of the wavelength. Generating a rapidly changing (<100 μs) sequence of pulses with freely selectable wavelengths, which is desired for applications, e.g. in spectroscopy, or for pump-sample experiments, may thus be realized only with difficulty or not at all.
FOPOs that are based entirely on fiber optics and that may be tuned electronically using the selection of the repetition rate for the pump laser are free of the drawbacks cited above. The FOPOs known from the prior art are limited, however, with respect to free configuration of the spectral bandwidths of the emitted pulses. In the past, in particular it has not been possible to generate impulses with very wide spectra, that is, temporally very short pulses, due to a low amplification bandwidth for the amplification media and fiber-induced dispersion effects. For example, to date it has only be possible to generate pulses having a duration of <500 fs in laser systems that used a glass fiber as amplifying medium, but that apart from this were constructed to prevent additional dispersion completely in free beam optics. Moreover, changing from femtosecond operation to picosecond operation is not possible or is only possible with significant modifications to the laser system.